如何在Scheme中将let作为lambda函数实现 [英] How to implement let as a lambda function in Scheme

问题描述

作为练习，我试图将let定义为lambda函数，如下所示:

(define let_as_lambda 
  (lambda (var) 
     (lambda (value body) 
       (var body) val)))

我希望这样称呼它:

((let_as_lambda a) (3 (+ a 2)))

但是，无法将未绑定的变量(在本例中为"a")作为参数传递给函数. (我知道它看起来有些奇怪，但是我需要let_as_lambda(var)返回一个函数.)

谁能告诉我该怎么做?任何建议表示赞赏.

实际上，仅使用此lambda等效表达式:

(let ((p1 v1) (p2 v2)...) body) = ((lambda (p1 p2...) body) v1 v2...)

我什至无法使它正常工作

(define let_as_lambda 
    (lambda (var val body) 
      ((var body) val)))

呼叫者:(let_as_lambda a 3 (+ a 2))

没有收到相同的投诉:

在定义标识符之前先引用标识符:a

解决方案

let是根据lambda定义的语法扩展.我认为您不能将其定义为函数.看看方案编程语言

As an exercise I am trying to define let as a lambda function something like this:

(define let_as_lambda 
  (lambda (var) 
     (lambda (value body) 
       (var body) val)))

And I am hoping to call it like this:

((let_as_lambda a) (3 (+ a 2)))

However there is no way to pass an unbound variable (in this case "a") as an argument to a function. (I know it looks a little strange but I need let_as_lambda(var) to return a function.)

Can anyone show me how to do this? Any advice is appreciated.

In fact, just using this lambda-equivalent expression:

(let ((p1 v1) (p2 v2)...) body) = ((lambda (p1 p2...) body) v1 v2...)

I can't even get this to work:

(define let_as_lambda 
    (lambda (var val body) 
      ((var body) val)))

Called by: (let_as_lambda a 3 (+ a 2))

Without getting the same complaint:

reference to an identifier before its definition: a

解决方案

let is a syntatic extension defined in terms of lambda. I don't think you can define it as a function. Take a look at the example from The Scheme Programming Language

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